1. Technical Field of the Invention
The present invention relates to a data transmission apparatus, and in particular, to a data transmission apparatus which is capable of performing good communication with a non-contact data carrier even in a situation where a pulse noise is generated.
2. Related Art
A variety of kinds of data transmission apparatuses have been known. As one example, Japanese Patent Application Laid-Open Publication No. 10-294684 discloses a data transmission apparatus capable of performing normal data communication under the circumstances where external noise is caused intermittently. In order to perform normal data communication, this apparatus amplifies and detects noise received by an antenna to detect the noise level while no signal is transmitted from a transmitting antenna coil. When the noise level is low, the transmission is started, and when the noise level becomes high, the transmission is not started until the noise level becomes low.
However, the foregoing publication is for such noise as background noise, for example, of this literature. Therefore, difficulty arises with such a technique in making communication under the circumstances where a pulse noise is caused within an extremely short period of time. For example, as shown in FIG. 16, let us assume that pulse noise is detected as having been caused (see FIGS. 16(c) and 16(d) under the circumstances where a motor is used (see FIG. 16(a)). Upon reception of command (read command or write command) transmission instructions from a host device (see FIG. 16(b)), a reader/writer recognizes, during the time when no pulse noise is caused, that the noise level has been reduced and starts transmission for an IC card (see FIG. 16(e)). Then, a pulse noise that is produced at the time instant of return transmission from the IC card may interfere with the return transmission (see FIGS. 16(c) and 16(f)) it is highly possible that it will cause a transmission error.